I had some really good responses, and I’m (relatively) certain that my plans to create a BP sawed-off are perfectly legal.

I figure it’s one of those things where I should just do it now before the fed decides that “due to their ability to circumvent traditional background checks, muzzleloading, and all blackpowder weapons previously not classified as firearms will be viewed; for all legal purposes, the same as cartridge firearms”.

Of course, the government would never write a law that was that easy to read.

Potato guns are not firearms federally, but New Jersey considers them firearms. They aren’t firearms federally because they don’t use fixed ammunition. It is the same designation as a black powder cannon.

Well, the law isn’t so much about propellant type as it is about whether the “fixed cartridge” or “fixed shotgun shell” attribute, at least for destructive device (if of an older design), any other weapon, or barrel length requirements.

Note that machine guns are excluded from this exemption : the law states that any weapon capable of firing more than one shot, without manual reloading, per pull of the trigger, regardless of ammunition — this could even include a lot of automatic electric airsoft or bb guns if the judge finds them to be a weapon.

As for propane, it wouldn’t work too well. The stuff requires oxygen to burn, and tends to get a little unpleasant once it’s mixed right. You could do it, but once you finish it’d be closer to an engine than a muzzleloader, and there wouldn’t be many advantages compared to simply purchasing Pyrodex pellets and setting up a feeder system with that.
It’s unfortunately the case for most liquid and gaseous energy sources. They either don’t ignite without further mixing, tend to be incredibly unstable (see nitroglycerin), and/or just plain unstable.

Except air guns are not firearms. I mean, you could mold something out of clay that looked a lot like a gun from more than a few yards away. If you start severely regulating (which is what the provisions of the National Firearms Act which regulate machine guns, sawed off shotguns, and short barreled rifles do) items that might look like firearms, but aren’t, it can get pretty ridiculous pretty fast.